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Method of and apparatus for evaluating the performance of a control systemRelated Patent Categories: Refrigeration, Automatic ControlMethod of and apparatus for evaluating the performance of a control system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050284160, Method of and apparatus for evaluating the performance of a control system. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] The present description relates generally to methods of and apparatuses for evaluating the performance of a control system. More specifically, the present description relates to automated methods of and apparatuses for detecting and diagnosing oscillations in a feedback control loop. [0002] Oscillating feedback control loops are a common occurrence in many types of control systems, such as heating, ventilating, and air conditioning (HVAC) systems. Oscillations are typically the result of periodic changes that cause a signal within a feedback control loop to vary undesirably in a deterministic and repeatable fashion. Oscillating loops are undesirable because they increase variability in the quality of control and product performance, accelerate equipment wear, and may cause further oscillations in other interacting loops. Thus, detection, diagnosis, and correction of oscillations are important activities in feedback control loop supervision and maintenance. [0003] Common causes of oscillations in feedback control loops include external oscillating disturbances, bad controller tuning, nonlinearities in a system component, such as static and dynamic nonlinearities, stiction (i.e., a sticking valve), incorrect component sizing, and poor actuator resolution, or a combination of these causes. Causes such as static and dynamic nonlinearities, stiction, incorrect component sizing, and poor actuator resolution are classified as system problems. External oscillating disturbances and bad controller tuning are classified as control problems. [0004] Oscillation diagnosis involves distinguishing between these different causes of oscillations once they have been detected. Distinguishing between these types of causes is important for identifying the appropriate remedial action. For example, attempts at re-tuning a controller will be unsuccessful or inefficient where a system problem is the cause of the oscillations rather than a control problem. [0005] One problem with existing methods of detecting oscillations is that they require prior specification of an expected time parameter for the feedback control loop. Usually the information is required to configure a noise filter, or to set detection thresholds. This information is often unavailable or cannot be reliably estimated. Further, existing methods of diagnosing oscillations typically require a manual visual inspection of the shape of several different signals in the oscillating feedback control loop in order to distinguish between control problems and system problems. These requirements increase the required effort and costs associated with implementing the diagnosis method. [0006] Thus, there is need for a method of and apparatus for evaluating the performance of a control system which does not require prior information about the feedback control loop being monitored. There is also need for a method of and apparatus for evaluating the performance of a control system that does not require monitoring shapes of multiple signals in the feedback control loop. There is further need for a method of and apparatus for evaluating the performance of a control system that is automated such that it does not require a manual visual inspection of a signal to distinguish among different causes of oscillations. There is yet further need for a method of and apparatus for evaluating the performance of a control system that is intuitive, easily implemented either directly or remotely, and requires minimal computational effort. SUMMARY [0007] According to an exemplary embodiment, a method of evaluating the performance of a control system includes calculating a ratio of a first area associated with the signal and a second area associated with the signal. The method also includes comparing the ratio with a predetermined value and determining a cause of an oscillation in the control system based on the comparison. [0008] According to another exemplary embodiment, an apparatus for evaluating the performance of a control system includes a processor operable to execute a first function configured to calculate a ratio of a first area associated with the signal and a second area associated with the signal. The first function is also configured to compare the ratio with a predetermined value and determine a cause of an oscillation in the control system based on the comparison. [0009] According to another exemplary embodiment, an apparatus for evaluating the performance of a control system includes means for calculating a ratio of a first area associated with a signal from the control system and a second area associated with the signal, means for comparing the ratio with a predetermined value, and means for determining a cause of an oscillation in the control system based on the comparison. [0010] According to another exemplary embodiment, a method of evaluating the performance of a control system includes detecting an oscillation in the control system. The method also includes calculating a ratio of a first area associated with a signal from the control system and a second area associated with the signal, comparing the ratio with a predetermined value, and determining a cause of an oscillation in the control system based on the comparison. [0011] According to another exemplary embodiment, an apparatus for evaluating the performance of a control system includes a processor operable to execute a first function and a second function, wherein the first function is configured to perform the steps of calculating a ratio of a first area associated with the signal from the control system and a second area associated with the signal, comparing the ratio with a predetermined value, and determining a cause of an oscillation in the control system based on the comparison. The second function is configured to perform the step of detecting an oscillation in the control system. [0012] According to another exemplary embodiment, a method of evaluating the performance of a control system includes determining an autocorrelation from a signal from the controller, filtering the signal based on the autocorrelation, generating an autoregressive moving average (ARMA) model of the filtered signal, calculating a performance index value for the control system based on the model, and determining whether an oscillation is present in the control system based on the performance index value. [0013] According to another exemplary embodiment, an apparatus for evaluating the performance of a control system includes a processor operable to execute a first function. The first function is configured to perform the steps of determining an autocorrelation from a signal from the controller, filtering the signal based on the autocorrelation, and generating an autoregressive moving average (ARMA) model of the filtered signal. The first function is further configured to perform the steps of calculating a performance index value for the control system based on the model and determining whether an oscillation is present in the control system based on the performance index value. [0014] Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many modifications and changes within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications. BRIEF DESCRIPTION OF THE DRAWINGS [0015] The exemplary embodiments will hereafter be described with reference to the accompanying drawings, wherein like numerals depict like elements, and: [0016] FIG. 1 is a diagram which illustrates an apparatus for evaluating the performance of a control system according to an exemplary embodiment; [0017] FIG. 2 is a diagram which illustrates an exemplary feedback control loop which may be tested using the apparatus of FIG. 1; [0018] FIG. 3 is a flow diagram which illustrates a method of detecting oscillations according to an exemplary embodiment; [0019] FIG. 4 graphically illustrates exemplary areas before and after peaks in an error signal for oscillations caused by system problems, and in an error signal for oscillations caused by control problems; and [0020] FIG. 5 is a flow diagram which illustrates a method of distinguishing between oscillations caused by control problems and oscillations caused by system problems according to an exemplary embodiment. 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